But he said more information about methods used by the intelligence community should be publicly available.

He said the American people had the right to know what was being done on their behalf and "blind trust is a false currency".

Referring to the international nature of intelligence, he added: "We rely heavily on liaison relationships and don't see how we could do business without them."

Nick Warner, Director-General of the Australian Secret Intelligence Service, said some intelligence agencies could talk more candidly to each other than to diplomats to convey sensitive messages.

He also said Mosul and Raqqa may be re-taken from Islamic State of Iraq and the Levant, but warned there could be a continuing Sunni insurgency, its ideology having spread too far to be destroyed quickly.